When producing pre-foamed beads of polyolefin resins which are used for obtaining foamed articles in a mold from polyolefin resins , resin granules are pre-foamed without kneading and melting again, and therefore, a shape of the resin granule has a large influence on that of preformed beads. It happens sometimes that a large deformation occurs at the time of foaming. For this reason, resin granules are needed which have a small uniform shape, a constant or preferably small residual strain and which can become spherical after pre-foaming. Thus, raw resins are beforehand molded into small granules of a proper shape. As this molding method for resin granules, there are a strand cut method, under water cut method and the like.
In the case of the strand cut method , a residual strain is large at the time of foaming resin granules, and it's shape is largely deformed due to a big shrinkage in a direction of resin granule extrusion when pre-foamed beads are produced.
Therefore, in order to obtain the pre-foamed beads which have a small change in shape and a desired shape, the most proper conditions must be fulfilled exactly which are determined by seizing a relation between producing conditions or properties of raw resins and a ratio of shape change. However, it is practically difficult to fulfill these producing conditions stably and exactly and a yield is forced to be low. Moreover, there are some problems that a lot of works are needed even to a small change in raw materials, that a long space is necessary to set an equipment, and that productivity is comparatively low due to a limit for increasing strand numbers, and the like.
As the under water cut method, there are methods described, for example, in Unexamined Japanese Patent (JPA 61-195808) or Unexamined Japanese Patent (JPA 1-234212), the former discloses a method of producing a spherical granules of a thermoplastic resin which is characterized in that a resin temperature is adjusted so that a viscosity of a fused resin at a die entrance becomes between 1.times.10.sup.2 and 5.times.10.sup.4 poise and, further, an amount of discharge is adjusted to between 0.1 and 6.0 kg/hr when a fused resin is extruded into a cooling water from a nozzle of die which is attached at a tip of a molding machine and is granulated by being cut with a rotating cutter blade which is contacted with a die surface. These method are not necessarily sufficient because pre-foamed beads can not be obtained yet stably as large voids in the pre-foamed beads appear or a shape of the pre-foamed beads become non-spherical , though there are no large shrinkage in a direction of extrusion at the time of producing pre-formed beads as can be seen in a strand cut method. Thus, even though resin granules of a desired shape can be obtained at a glance, it happens sometimes that they cannot be used since large voids appear or their shape deforms largely when they were pre-foamed. In this case, losses of resin and time and labor for producing them become quite large since the fact that the resin granules are improper becomes clear long time after evaluating operations including a foaming operation which takes a long time succeeding to production of the resin granules. Further, since this tendency becomes remarkable as a discharge amount per one hole of a die nozzle increases, the application of this method to practical production scale is not necessarily satisfactory.
Pre-foamed beads are mixed if necessary, with compounding ingredients such as nucleating agents, anti-fusing agents , anti-static agents, pigments and the like. However, since the dispersion of these compounding ingredients is not necessarily sufficient due to small quantity of these compounding ingredients, properties of the obtained mixtures such as an uniformity in cell size and coloring, which is one of important factors for determining properties of a foamed article, are not always sufficient. Thus , a method is waited which can make a rapid progress in dispersibility of above-mentioned ingredients in the mixtures by a simple way.
Further, a single-screw extrusion is usually used. Recently, even in the case of a foamed article of polyolefin resins, a successive production of a variety of foamed articles, which are composed of plural resins or mixtures, is not unusual. Usually, these foamed articles are produced using the same equipment from a viewpoint of economical efficiency. However, when the single-screw extrusion is used, it is a weak point that productivity is quite low, since a good deal of time and labor are needed for switching of raw resins and a loss of raw materials is also large.
An object of this invention is to provide a producing method which makes it possible that the dispersion of compounding agents becomes uniform, that a switching of raw materials can be done smoothly and economically and further, that resin granules or pre-foamed beads of a desired shape can be produced by solving problems in the prior art as mentioned above.
The inventors have made a series of studies and finally have found a method which can solve the above-mentioned problems and have completed this invention by using a homodirectional (co-rotating) twin-screw kneading extruder as an extruder , extruding a resin by an under water cut method and adjusting a temperature of warm water in a proper range taking into consideration it's relation with a resin temperature etc., under conditions of a proper amount of discharge per one nozzle hole and a diameter of die nozzle which are determined by a desired shape of resin granules and an amount of production.